SANAA, June 28 : At least 21 people have died in Yemen after clashes between Houthi fighters and forces loyal to exiled President Abd-Rabbu Mansour Hadi in the cities of Aden and Taiz, reports say.Witnesses said 14 were killed when Houthi forces launched an assault on a major oil refinery in Aden on Saturday, firing missiles at storage tanks and starting a large fire that billowed over parts of the city, Reuters news agency reported.Video footage showed black smoke over the facility in Aden's Buraiqah district on Saturday. Earlier, an Al Jazeera source confirmed the attack, and reported that at least one person - an employee at the refinery - was killed.Both the port and the refinery in Aden are controlled by pro-government fighters, and the area has seen fierce clashes between rival forces.To the north in Taiz, fighting left at least seven Houthi fighters dead and 16 others injured, as the Iran-allied group targeted residential neighbourhoods using tanks and heavy artillery. There were no figures available on how many fighters loyal to the Yemeni government had been killed or wounded after the clashes in the Mount Jarah district of the city.Houthi shelling also killed a Saudi soldier and wounded another in Jizan province bordering Yemen, according to the Saudi interior ministry.In other developments, a Hadi government official told AFP news agency that the Houthis targeted a Qatari vessel carrying food supplies from Djibouti, a hub for Yemen-bound humanitarian aid, and that the attack forced it to turn back.The Houthis have been battling the Yemeni government for months, taking control of major cities, including the capital Sanaa, and forcing Hadi into exile in neighbouring Saudi Arabia. An Arab coalition, which includes Saudi Arabia, has launched air strikes aimed at restoring Hadi. ?Al Jazeera and Agencies